Scientists cool buildings by beaming heat into space

Ready to give expensive air conditioning bills the cold shoulder? Researchers at Stanford have found a way to keep buildings cool using a new super-thin material that can beam indoor heat into space. “Cooling is very important,” Project Leader and Stanford Professor Shanhui Fan said. “It’s one of the major energy drivers in buildings, and we’d been thinking about whether we could use outer space, which is very cold, for cooling purposes.”

Daytime radiative cooling is a hurdle that, until recently, engineers have been unable to overcome. In the past, reflectors would absorb too much sunlight, causing the mirrors to heat up and lose any cooling capabilities. Another problem was radiating the heat back into space, which must be done within a specific wavelength or else the Earth’s atmosphere will reflect the light back down.

To get around these issues, Fan and his team built upon previous technology to create the material. “There is a technology, before our work, called nighttime cooling,” Fan told, “where basically you have a material that emits very well in the infrared, and that will cool itself down when you send the heat on the outside. What we did was figure out a way to combine [nighttime cooling technology] with a very good reflector for sunlight, so that it doesn’t [absorb] heat [from] the sun.”

The new cooling material consists of seven layers of silicon dioxide and hafnium dioxide that is laid upon a sheet of silver, creating a panel 1.8 microns thick -  thinner than a human hair. This panel acts as an extremely efficient mirror, which can reflect 97 percent of sunlight while radiating heat back into space. “It does two things at the same time,” Fan said. “It reflects sunlight, and it emits, very strongly, thermal radiation.” This approach uses outer space as a “heat sink,” turning the universe into a never-ending dumping ground for Earth’s heat. While nighttime cooling uses the same technique, according to Fan, “Our innovation is to be able to do this during the day when the sun is shining.”

The panels would then be placed upon a building’s roof to cool down the structure, greatly reducing the owner’s electric bill.  For example, a typical one-story house with 10 percent of its roof covered could cut its air-conditioning needs down by 35 percent during the most sweltering summer hours. “Essentially, I think that if you cover a significant portion of the roof, in many buildings you can significantly offset the cooling load,” Fan says. Not only would this cut electricity costs, it would lessen the consumption of fossil fuels, softening the environmental impact. The technology could also be utilized in off-grid areas as well, cooling down houses in third-world countries. However, Fan says the device works best with larger structures, such as office buildings. “The ideal building is one that has a large roof space, since the cooling power scales with area.”

So when can we expect to save some bucks on air conditioning? According to Fan, it may be only a few years away. “We’re trying to build a small system for prototyping. It should take a year or two, and then we’ll go from there.”

Fan and his team published their findings on Nov. 27 in the journal Nature.